Tonnage monitor for a mechanically driven press

ABSTRACT

The upper platen of a press is driven by a ballscrew that is rotated by an electric motor. The ballscrew is suspended from the crown of the press, and the force on the ballscrew shaft when the upper platen is driven into the workpiece causes the crown to flex upward. The flexure of the crown is sensed by a position transducer mounted on a cantilever support. The position transducer generates a signal that can be used to determine the tonnage force developed by the press.

FIELD OF THE INVENTION

[0001] The invention relates generally to developing a signal representative of the force exerted by a mechanically driven press through the use of a position sensing transducer.

BACKGROUND OF THE INVENTION

[0002] Presses used for metal forming operations such as hemming are well known in the art. In order to accommodate automotive body panels, hemming presses have a footprint on the order of 125 square feet, and are 16-20 feet tall. Such presses typically use a hydraulic cylinder to raise and lower the upper platen of the press, and exert a force on the workpiece exceeding 200,000 pounds. High tonnage presses are normally driven by a hydraulic cylinder, but facilities that use such presses are desirous of replacing the hydraulic drive on a press with a mechanical drive such as a ballscrew. A mechanical drive does not use hydraulic fluid, will not develop hydraulic fluid leaks, and does not present hydraulic fluid disposal problems. As a result, mechanical drives are more environmentally friendly than hydraulic drives. Mechanical drives have other advantages over hydraulic drives; a mechanical drive consumes less energy than a hydraulic drive, a mechanical drive is quieter in operation than a hydraulic drive, a mechanical drive is more reliable than a hydraulic drive, and a mechanical drive can be designed with positive positioning and positive position holding features.

[0003] An electric motor with an encoder can be controlled to rotate a prescribed number of turns in an attempt to produce a predicted force on a workpiece. The exact force that is actually produced however is dependent on a number of variables such as the operating temperature of the press and the dies, the temperature of the workpiece, the amount of wear on the press and the dies, the weight of the dies, any high spots on the workpiece, two panels stuck together, debris and other variables that are impossible to predict or control.

[0004] Thus, it would be desirable to provide a mechanism that would provide an indication of the magnitude of the force or the tonnage that is actually developed by a mechanically driven press. The developed tonnage could be displayed on a gauge or on a screen and monitored by an operator to ensure that the tonnage on a workpiece is not too high or too low, ensuring part repeatability and quality. In the case of a hydraulically driven press, it is a simple matter to couple a pressure transducer to the hydraulic fluid used to drive the press platen, and the tonnage exerted by the platen will be related to the pressure of the hydraulic fluid as sensed by the pressure transducer. In the case of a mechanically driven press however, there is no hydraulic fluid, and a different method of monitoring and reading the tonnage exerted by the press has to be devised.

[0005] It would accordingly be desirable to provide a tonnage monitor for a mechanically driven press that would provide an indication of the tonnage exerted by the press during press operation.

SUMMARY AND OBJECTS OF THE INVENTION

[0006] The invention provides a tonnage monitor for a mechanically driven press. An electric motor is used to drive a ballscrew shaft that is suspended from the crown of the press. The upper press platen is mounted on the ballscrew nut, and rotation of the ballscrew shaft causes the upper press platen to be driven up and down. The crown of the press is designed to flex as the upper press platen applies increasing force against the workpiece, and the flexure is sensed by a transducer and used to provide an indication of the tonnage developed by the press.

[0007] It is accordingly an object of the invention to provide a press with a mechanical drive that includes a tonnage monitor to provide an indication of the tonnage developed by the press.

[0008] It is another object of the invention to provide a press that is driven by a ballscrew suspended from the crown of the press in which the deflection of the crown that occurs during operation of the press is used to provide an indication of the tonnage force developed by the press.

[0009] It is another object of the invention to provide a mechanically driven press in which a displacement transducer is used to measure deflection of the crown of the press in order to develop a signal representative of the tonnage force that is developed by the press.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiment and accompanying drawings in which:

[0011]FIG. 1 is a perspective view of a mechanically driven press according to the invention showing the upper platen of the press in the Open position;

[0012]FIG. 2 is a detail view of the press of FIG. 1 showing the crown of the press when the upper platen of the press in the Open position;

[0013]FIG. 3 is a detail view of the press of FIG. 1 showing the crown of the press when the upper platen of the press is in the Closed position exerting a force on the lower press platen;

[0014]FIG. 4 shows a load cell in a test fixture used to calibrate the output of a displacement transducer; and,

[0015]FIG. 5 is a graph showing the relationship of the tonnage developed by the press to the voltage output of the displacement transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] Turning now to the drawings, FIG. 1 shows a mechanically driven press generally designated by the reference numeral 10. The press comprises a lower base 12 that supports a lower platen 13. A lower die 14 is positioned on the lower platen 13 and global change the lower die 14 is configured to support a workpiece 16. Two vertical columns 18 are mounted on diagonally opposite corners of the lower base, and the two corner posts support the crown 19 of the press. The crown of the press is a box beam comprising a top plate 21 and a bottom plate 22 spaced from one another by two side plates 23 (only one shown). Two end plates 24 (only one shown) are provided one each at each end of the crown 19 to close the ends of the box beam and to prevent the introduction of foreign matter to the interior of the beam. Each corner post houses a vertical tie bar 25 (best seen in FIG. 2) that extends from the base of the press and extends out of the top plate 21 of the crown. A tie bar nut 27 is threaded onto the top of each tie bar 25 to secure the crown 19 of the press to the respective corner post 18. The tie bar nuts 27 are tightened to preload the corner posts 18 with a combined preload force that is greater than the largest force that will normally be exerted by the press on a workpiece.

[0017] An electric motor 30 that is used to drive the press is mounted on a support plate 32 that is cantilevered from a spacer block 33. The spacer block 33 is positioned in the middle of the crown 19, equally spaced from the two tie bar nuts 27. A mounting bracket 35 is attached to the cantilevered end of the support plate 32 remote from the spacer block 33, and a displacement transducer 36 such as a Linear Variable Differential Transformer (LVDT) is mounted to the support plate 32 by the bracket 35. The LVDT 36 comprises a body and a spring loaded probe 38 that extends from one end of the body. The tip of the spring loaded probe 38 is in contact with the top plate 21 of the crown of the press. The tip of the probe 38 will remain in contact with the top plate 21 of the crown of the press in the event that the crown of the press moves relative to the body of the LVDT. A signal lead 39 couples a signal from the LVDT 36 to an output display on the control panel of the press (not shown).

[0018] The electric motor 30 that drives the press includes a built-in encoder 41 and has an output shaft that is coupled to a clutch 42. The clutch 42 is coupled to a gearbox 44 and a brake 46 is mounted on the side of the gearbox that is opposite the clutch 42. The output of the gearbox 44 is coupled to a vertical drive shaft 50 that extends out of the bottom of the gearbox. The vertical driveshaft 50 from the gearbox is keyed to a drive socket 51 formed on the upper end of a thrust shaft 52. Two thrust bearings 54 are used to mount the thrust shaft 52 for rotation in the crown 19 of the press. The thrust shaft 52 and the thrust bearings 54 isolate the gearbox 44 from the compressive forces that the ballscrew generates as the upper platen and die are driven by the ballscrew into the lower platen and die. A mounting flange 56 on the lower end of the thrust shaft 52 is attached by bolts to an upper gimbal joint 58 that is connected to the shaft 59 of a ballscrew. A ballscrew nut 61 is threaded on the lower end of the ballscrew shaft 59, and the ballscrew nut is connected by a lower gimbal joint 62 to the upper platen 64 of the press. The upper platen 64 is formed with a vertical platen tube 65 that can receive the lower end of the ballscrew shaft when the platen is in the raised position. An upper die 67 is mounted on the underside of the upper platen 64. In order to guide the vertical motion of the upper platen 64, a hollow sleeve 68 is mounted on each corner of the upper platen and each hollow sleeve is in sliding engagement with one of four guide posts 70 that are mounted one each on the four corners of the press base.

[0019] In operation, the electric drive motor 30 rotates the ballscrew shaft 59 to reciprocate the upper press platen 64 and upper die 67 between Open and Closed positions. The rotation of the motor 30 is monitored by the encoder 41 to rotate the ballscrew shaft the required number of revolutions to drive the upper press platen from the Open position to the Closed position and to exert the required force on the workpiece 16. When the upper platen 64 is in the Open position as shown in FIG. 1, the upper platen is raised, and a workpiece 16 may be placed on or removed from the lower die 14 on the lower platen 13. FIG. 2 is a detail view showing the crown 19 of the press and the LVDT 36 When the upper press platen is in the Open position. The top plate 21 of the crown of the press is flat, and the signal output of the LVDT 36 when the crown is in this condition can be calibrated to be zero.

[0020] When the upper platen 64 is in the Closed position, the upper die is in contact with the workpiece 16. Further rotation of the ballscrew shaft 59 after the upper platen and die have come into contact with a workpiece will increase the pressure on the workpiece, and the upper and lower dies will compressively form the workpiece into the desired configuration as is well known in the art. The increased pressure on the workpiece is transmitted by the ballscrew shaft 59 through the upper gimbal 58 to the thrust bearings 54. The thrust bearings transmit the force on the ballscrew shaft 59 to the crown 19 of the press, causing the crown to flex upward into a curved shape as shown in FIG. 3.

[0021]FIG. 3 is a detail view showing the crown 19 of the press and the LVDT 36 when the upper press platen is in the Closed position, exerting a force on the lower press platen. The force exerted by the upper platen on the lower platen causes the crown to flex upward into a curved shape. At the same time, the spring loaded probe 38 of the LVDT 36 extends out of the LVDT housing to follow the motion of the crown. The dotted line 72 shows the displacement of the center of the crown 19 of the press relative to the ends of the tie bars 25 when the crown is flexed upward into a curved shape. The motion of the spring loaded probe 38 causes the LVDT to develop a second output signal that is representative of the tonnage force exerted by the press.

[0022] In actual practice, the LVDT used is a Schaevitz model GCD-SE. The LVDT has a signal output of 0-5 volts DC, and the spring loaded probe 38 has a stroke of 0.25 inches. In order to calibrate the output of the LVDT so that the output can be converted to the tonnage force exerted by the press, a load cell 75 can be mounted in the press as shown in FIG. 4. The load cell 75 is positioned on a test stand 74, and the test stand 74 is placed on the base 12 of the press. An upper test fixture 76 is mounted on the upper platen 64, in direct vertical alignment with the load cell 75. The press motor is then energized to drive the upper test fixture 76 into the load cell 75, and the load cell will measure the force exerted by the press. At the same time, the voltage output of the LVDT can be measured throughout the tonnage range of the press, and a calibration curve such as the curve 78 shown in FIG. 5 can generated. The constant slope of the curve 78 of FIG. 5 indicates that the relationship between the force measured by the load cell and the output voltage of the LVDT is constant.

[0023] Once the curve 78 of FIG. 5 has been generated, the slope of the curve, in this case 58.78 tons/volt, can be used to convert the voltage signals from the LVDT 36 to an indication of the tonnage force generated by the press. The tonnage force signals can be displayed on an output display such as a video monitor on the control panel of the press in a manner well known to those skilled in the art. In tests using a press of the type shown in FIG. 1, the deflection of the crown of the press measured by the LVDT varied from zero to 0.080 inches as the tonnage of the press increased from zero to 100 tons.

[0024] Having thus described the invention, various alterations and modifications will be apparent to those skilled in the art, which alterations and modifications are intended to be within the scope of the invention as defined by the appended claims. 

We claim:
 1. A mechanically driven press having a tonnage monitor, the press comprising: a base having a lower platen for supporting a workpiece; a pair of vertical columns attached to the base and a crown supported by the columns; a platen driving member suspended from the crown and coupled to the upper platen; a drive source for causing the platen driving member to press the upper platen against the workpiece; and, a displacement measuring instrument for measuring the distortion of the crown caused by the platen driving member pressing the upper platen against the workpiece.
 2. The mechanically driven press of claim 1 further comprising: a ballscrew comprising the platen driving member; and, an electric motor driving the ballscrew comprising the drive source.
 3. The mechanically driven press of claim 2 further comprising: a body and a movable probe comprising the displacement measuring instrument; and, a cantilever support for the displacement measuring instrument for measuring the distortion of the crown, whereby the cantilever support isolates the body of the displacement measuring instrument from the distortion of the press crown.
 4. The mechanically driven press of claim 3 further comprising: a displacement transducer comprising the displacement measuring instrument.
 5. The mechanically driven press of claim 4 further comprising: a linear variable differential transformer (LVDT) comprising the displacement transducer.
 6. The mechanically driven press of claim 5 further comprising: a mount for mounting the body of the LVDT to the cantilever support; and, a probe extending from the body of the LVDT, whereby the end of the probe is in contact with the crown of the press, and whereby movement of the crown of the press relative to the cantilever support causes the probe to move relative to the body of the LVDT.
 7. The mechanically driven press of claim 6 further comprising: A block for mounting the cantilever support to the crown of the press, the block being attached to the crown of the press at the middle of the crown, midway between the pair of vertical columns.
 8. A press comprising an upper platen and a lower platen and having a tonnage monitor for monitoring the force exerted by the upper platen against the lower platen, the press comprising: a base for supporting the lower platen; a pair of vertical columns attached to the base and a crown supported by the columns; a platen driving member suspended from the crown and coupled to the upper platen; a drive source for causing the platen driving member to press the upper platen against the lower platen; guide posts for maintaining the upper platen in alignment with the lower platen; and, an instrument for measuring the distortion of the crown caused by the platen driving member pressing the upper platen against the lower platen.
 9. The press of claim 8 further comprising: An electric motor comprising the drive source; a ballscrew comprising the platen driving member; a thrust shaft mounted for rotation in the crown of the press, the thrust shaft being coupled to one end of the ballscrew; and, bearings for mounting the thrust shaft in the crown of the press, whereby force developed by the press is transferred by the ballscrew to the crown of the press and the resulting distortion of the crown is measured by the instrument for measuring the distortion of the crown.
 10. The mechanically driven press of claim 9 further comprising: a linear variable differential transformer (LVDT) having a motion following probe extending from the body of the LVDT in contact with the crown of the press, the LVDT comprising the instrument for measuring the distortion of the crown; and, a cantilever support for the LVDT, whereby the distortion of the crown of the press is isolated from the body of the LVDT and the probe of the LVDT is able to follow the distortion of the crown.
 11. The mechanically driven press of claim 10 further comprising: A block for mounting the fixed end of the cantilever support to the crown, the block being located in the middle of the crown.
 12. The mechanically driven press of claim 11 wherein the force developed by the press is at least 150,000 pounds.
 13. The mechanically driven press of claim 12 wherein the distortion of the crown of the press that is measured by the LVDT is less than 0.25 inches.
 14. A press comprising an upper platen and a lower platen and having a tonnage monitor for monitoring the force exerted by the upper platen against the lower platen, the press comprising: a base for supporting the lower platen; a pair of vertical columns attached to the base and a crown supported by the columns; a ballscrew shaft suspended from the crown and coupled to the upper platen; a rotary electric motor coupled to the ballscrew shaft for causing the platen driving member to press the upper platen against the lower platen; guide posts for maintaining the upper platen in alignment with the lower platen; an instrument for measuring the distortion of the crown caused by the platen driving member pressing the upper platen against the lower platen; and, a cantilever support supporting the instrument over the top of the crown, the cantilever support being mounted on a support block, the support block being located in the center of the crown, whereby the instrument measures the increase in height of the center of the crown relative to the ends of the crown. 